The present invention relates, in general, to waste water chlorination techniques and, in particular, to a new and useful breakpoint chlorination method and apparatus which regulates the dosing of waste water with chlorine according to a derivative of chlorine residual with respect to the chlorine dose.
A breakpoint chlorination operation removes ammonia (NH.sub.3) from waste water by oxidizing the ammonia to nitrogen gas (N.sub.2), water and hydrochloric acid (HCl). Mechanically agitated tanks mix the waste water with chlorine gas (Cl.sub.2) and a solution of base to neutralize the HCl. In a small reactor, the NH.sub.3 and Cl.sub.2 reaction occurs rapidly. Then a larger reactor allows residual Cl.sub.2 to disinfect the waste water. In a final process step, sulfur dioxide (SO.sub.2) dechlorinates the process stream. Such a process requires the control of chlorine concentration, pH and final dechlorination.
The Cl.sub.2 remaining in the water stream after the reaction depends on the amount of Cl.sub.2 added and the NH.sub.3 concentration in the influent. The relationship between these variables is nonlinear. The nonlinearity appears as a variable gain that can be positive or negative, thus complicating the control problem.
In the chlorination of waste waters and the subsequent treatment thereof, it is known to add desired quantities of chlorine, base and sulfur dioxide by utilizing controllable valves which are controlled according to one parameter or another of the treatment apparatus, for example, liquid or gas flow.